The invention relates to vapor generation apparatus and particularly to apparatus for connecting the buckstay structure to the tube wall. While the invention has particular application to spiral tube wall furnace constructions it will be understood that it also has application to vertical tube furnace apparatus.
Vapor generation apparatus such as large utility boilers that are disposed in a frame that is provided to withstand the internal furnace gas pressure. As the furnace approaches operating temperature, the furnace walls expand vertically and horizontally. Additionally, the pressure excursions within the furnace, either an increase or a decrease in pressure within the furnace, cause a resultant additional flexing of the tube walls either inwardly or outwardly in a horizontal direction.
It is customary in the art to locate buckstays in the form of I-beams adjacent opposite walls of the furnace so that the buckstays can be attached to the furnace wall to maintain the adjacent wall in a flat condition. Typically both vertical and horizontal structural members are used that are respectively known as vertical and horizontal buckstays.
Since the furnace walls change in temperature from ambient to temperatures in the order of 400 degrees Centigrade, while the buckstays are at ambient temperature essentially at all times, it is essential that provision be made to permit appropriate expansion. This is accomplished by carrying the buckstays on the wall in a manner which permits the vertical expansion of the walls. The opposing buckstays are tied to each other through connections which are at the boiler wall temperature.
Typically, the horizontal buckstays are disposed in bands around the perimeter of the furnace tube walls at vertically spaced intervals (often between 15 and 40 feet) throughout the height of the furnace wall. Horizontally, the buckstays on opposite walls of the furnace are interconnected through buckstay ties so that the reaction of one buckstay is resisted by the reactions of the buckstay on the opposing wall so it can counteract the pressure forces acting on the furnace walls. It has been customary to provide vertical support members to interconnect adjacent buckstays with a connection that permits a sliding action that permits relative movement between the furnace tube wall with which a buckstay cooperates and the buckstays themselves. As the furnace expands in a vertical direction the effect on the various levels of buckstays will be different.
Because of the temperature differential between the furnace wall and the buckstays it is preferred to locate the buckstays a short distance from the furnace wall with insulation therebetween. Apparatus referred to as stirrups are used to make this connection between the furnace wall and the buckstay. The stirrup is a device known in the art which allows gas pressure loading to be transmitted from the furnace tube walls to the buckstay system while allowing unrestricted thermal expansion of the boiler tube wall envelope. Known stirrup constructions are disclosed in U.S. Pat. No. 4,395,860, and 4,059,075. These stirrups may be connected to the furnace wall and operate to support the buckstay as well as to prevent relative inward or outward movement between the buckstay and the wall.
The prior art designs are not wholly satisfactory because they will not transmit loading due to negative furnace gas pressure loading. In addition, the conventional design must be attached to a flat surface. Still another disadvantage of most designs is that there is insufficient contact area for connection to the buckstay for bearing requirements. Still another disadvantage of the present design is that it attaches to the boiler support plates and this causes an undesirable additional loading on the boiler support plates.
The principle of the spiral wall helical wall furnace is to increase the mass flow per tube by reducing the number of tubes needed to envelope the furnace without increasing the spacing between the tubes. This is done by arranging the tubes at an angle and spiralling them around the furnace.
It is customary in spiral tube wall furnaces to provide the spiral tube construction in only the lower elevational portion of the furnace. The walls are of plate and tube construction meaning that the a plurality of spirals of tubes are substantially coplanar and adjacent axial sections are joined by an intermediate fin. Typically the spiral tube is disposed at angle with respect to a horizontal plane and the angle is typically between 7 and 30 degrees. Without additional structure to support the tubing, the entire weight of the wall would be supported by welds intermediate the respective fins and tubes.
To avoid placing the weight on the welds between respective fins and axial sections of tubing it is customary to provide support plates that span a plurality of tubes to avoid the loading on the fins and tubes that may even flatten the normally round tubes.
It is desirable to connect the tube walls to the buckstays with stirrups. The stirrup allows relative motion between the tube wall the buckstay system.